This invention relates generally to an overload control circuit for a DC motor and more particularly to a digital control circuit including means providing programmable time relationships between input armature current which affects motor temperature and multiple digital output signals which are adapted to signal overload alarm and trip conditions.
Load overheating from electrical resistance type losses is of particular concern for motor and other similar type loads since the overheating increases generally as the square of the current drawn thereby. As a consequence, various types of protective means have been employed in the past for disconnecting the motor from the power source before damage can occur. While various types of electromechanical overload trip relay type circuitry have been employed, more recently, solid state devices and digital implementations are being utilized. The following compilation represents teachings of prior art practice of solid state implementations of motor overload protective circuitry: U.S. Pat. No. 3,735,219, "Thermal Protection Device For Linear Motor", Kahn, et al., May 22, 1972; U.S. Pat. No. 3,845,354, "Solid State Thermal Overload Indicator", Boothman, et al., Oct. 29, 1974; U.S. Pat. No. 3,927,359, "Engine Starter Motor Control For Preventing Damage During Hydraulic Lock", Chen, Dec. 16, 1975; U.S. Pat. No. 4,021,700, "Digital Logic Control System For Three Phase Submersible Pump Motor", Ellis-Anwyl, May 3, 1977; and U.S. Pat. No. 4,041,540, "Solid State Overload Relay Circuit", Kamph, et al., Aug. 9, 1977.
The aforementioned U.S. Pat. No. 4,021,700 demonstrates that prior art control systems are relatively complex and are of costly construction requiring considerable space while consumming substantial power. In contrast, the system disclosed in U.S. Pat No. 3,845,354 achieves its desired results by means of a low cost, low power, highly efficient and reliable control system that can be contained within a very small fraction of the space needed by prior systems. It is to be noted that the present invention also achieves the desired results in yet an improved manner.